=======================
RoverDude 1:
========================
using RUI.Icons.Selectable;
using System;
using System.Collections.Generic;
using UnityEngine;

namespace USITools
{
	[KSPAddon(KSPAddon.Startup.MainMenu, true)]
	public class USI_KolonyFilter : MonoBehaviour
	{
		private static System.Collections.Generic.List<AvailablePart> kolonyParts = new System.Collections.Generic.List<AvailablePart>();

		internal string category = "Filter by Function";

		internal string subCategoryTitle = "Kolonization";

		internal string defaultTitle = "UKS";

		internal string iconName = "R&D_node_icon_start";

		internal bool filter = true;

		private void Awake()
		{
			GameEvents.onGUIEditorToolbarReady.Add(new EventVoid.OnEvent(this.SubCategories));
			USI_KolonyFilter.kolonyParts.Clear();
			foreach (AvailablePart current in PartLoader.LoadedPartsList)
			{
				if (current.partPrefab && current.manufacturer == "USI - Kolonization Division")
				{
					USI_KolonyFilter.kolonyParts.Add(current);
				}
			}
		}

		private bool EditorItemsFilter(AvailablePart avPart)
		{
			return USI_KolonyFilter.kolonyParts.Contains(avPart);
		}

		private void SubCategories()
		{
			Icon icon = PartCategorizer.Instance.iconLoader.GetIcon(this.iconName);
			PartCategorizer.Category category = PartCategorizer.Instance.filters.Find((PartCategorizer.Category f) => f.button.categoryName == this.category);
			PartCategorizer.AddCustomSubcategoryFilter(category, this.subCategoryTitle, icon, (AvailablePart p) => this.EditorItemsFilter(p));
			RUIToggleButtonTyped activeButton = category.button.activeButton;
			activeButton.SetFalse(activeButton, RUIToggleButtonTyped.ClickType.FORCED);
			activeButton.SetTrue(activeButton, RUIToggleButtonTyped.ClickType.FORCED, true);
		}
	}
}
============================
RoverDude 2:
============================

using RUI.Icons.Selectable;
using System;
using System.Collections.Generic;
using UnityEngine;

namespace USITools
{
	[KSPAddon(KSPAddon.Startup.MainMenu, true)]
	public class USI_FreightFilter : MonoBehaviour
	{
		private static System.Collections.Generic.List<AvailablePart> kolonyParts = new System.Collections.Generic.List<AvailablePart>();

		internal string category = "Filter by Function";

		internal string subCategoryTitle = "Freight";

		internal string defaultTitle = "FTT";

		internal string iconName = "RDicon_fuelSystems-highPerformance";

		internal bool filter = true;

		private void Awake()
		{
			GameEvents.onGUIEditorToolbarReady.Add(new EventVoid.OnEvent(this.SubCategories));
			USI_FreightFilter.kolonyParts.Clear();
			foreach (AvailablePart current in PartLoader.LoadedPartsList)
			{
				if (current.partPrefab && current.manufacturer == "USI - Freight Division")
				{
					USI_FreightFilter.kolonyParts.Add(current);
				}
			}
		}

		private bool EditorItemsFilter(AvailablePart avPart)
		{
			return USI_FreightFilter.kolonyParts.Contains(avPart);
		}

		private void SubCategories()
		{
			Icon icon = PartCategorizer.Instance.iconLoader.GetIcon(this.iconName);
			PartCategorizer.Category category = PartCategorizer.Instance.filters.Find((PartCategorizer.Category f) => f.button.categoryName == this.category);
			PartCategorizer.AddCustomSubcategoryFilter(category, this.subCategoryTitle, icon, (AvailablePart p) => this.EditorItemsFilter(p));
			RUIToggleButtonTyped activeButton = category.button.activeButton;
			activeButton.SetFalse(activeButton, RUIToggleButtonTyped.ClickType.FORCED);
			activeButton.SetTrue(activeButton, RUIToggleButtonTyped.ClickType.FORCED, true);
		}
	}
}



============================
Infernal Robotics:
============================
using System.Collections.Generic;
using InfernalRobotics.Control.Servo;
using UnityEngine;

namespace InfernalRobotics.Gui
{
    [KSPAddon(KSPAddon.Startup.MainMenu, true)]
    public class IREditorCategory : MonoBehaviour
    {
        private static readonly List<AvailablePart> availableParts = new List<AvailablePart>();

        void Awake()
        {
            GameEvents.onGUIEditorToolbarReady.Add(IRCustomFilter);

            //create list of parts that have MuMechToggle module in them
            availableParts.Clear();
            availableParts.AddRange(PartLoader.LoadedPartsList.InfernalParts());
        }

        private void IRCustomFilter()
        {
            const string FILTER_CATEGORY = "Filter by Function";
            const string CUSTOM_CATEGORY_NAME = "Robotic Parts";

            var texture_on = new Texture2D(36, 36, TextureFormat.RGBA32, false);
            var texture_off = new Texture2D(36, 36, TextureFormat.RGBA32, false);

            InfernalRobotics.Utility.TextureLoader.LoadImageFromFile(texture_on, "icon_filter_on.png");
            InfernalRobotics.Utility.TextureLoader.LoadImageFromFile(texture_off, "icon_filter_off.png");
            RUI.Icons.Selectable.Icon icon = new RUI.Icons.Selectable.Icon("Infernal Robotics", texture_off, texture_on);

            //Adding our own subcategory to main filter
            PartCategorizer.Category filter = PartCategorizer.Instance.filters.Find(f => f.button.categoryName == FILTER_CATEGORY);
            PartCategorizer.AddCustomSubcategoryFilter(filter, CUSTOM_CATEGORY_NAME, icon, p => availableParts.Contains(p));

            RUIToggleButtonTyped button = filter.button.activeButton;

            button.SetFalse(button, RUIToggleButtonTyped.ClickType.FORCED);
            button.SetTrue(button, RUIToggleButtonTyped.ClickType.FORCED);
        }
    }
}
============================
KIS:
============================

using System;
using System.Collections.Generic;
using System.Linq;
using UnityEngine;

namespace KIS
{
    //using UnityEngine;

    [KSPAddon(KSPAddon.Startup.MainMenu, true)]
    public class KISAddonEditorFilter : MonoBehaviour
    {
        private static List<AvailablePart> avPartItems = new List<AvailablePart>();
        internal string category = "Filter by Function";
        internal string subCategoryTitle = "EVA Items";
        internal string defaultTitle = "KIS";
        internal string iconName = "R&D_node_icon_evatech";
        internal bool filter = true;

        void Awake()
        {
            GameEvents.onGUIEditorToolbarReady.Add(SubCategories);

            avPartItems.Clear();

            foreach (AvailablePart avPart in PartLoader.LoadedPartsList)
            {
                if (avPart.name == "kerbalEVA" || avPart.name == "kerbalEVA_RD" || !avPart.partPrefab) continue;
                ModuleKISItem moduleItem = avPart.partPrefab.GetComponent<ModuleKISItem>();
                if (moduleItem)
                {
                    if (moduleItem.editorItemsCategory)
                    {
                        avPartItems.Add(avPart);
                    }
                }
            }

        }

        private bool EditorItemsFilter(AvailablePart avPart)
        {
            if (avPartItems.Contains(avPart))
            {
                return true;
            }
            else
            {
                return false;
            }
        }

        private void SubCategories()
        {
            RUI.Icons.Selectable.Icon icon = PartCategorizer.Instance.iconLoader.GetIcon(iconName);
            PartCategorizer.Category Filter = PartCategorizer.Instance.filters.Find(f => f.button.categoryName == category);
            PartCategorizer.AddCustomSubcategoryFilter(Filter, subCategoryTitle, icon, p => EditorItemsFilter(p));

            RUIToggleButtonTyped button = Filter.button.activeButton;
            button.SetFalse(button, RUIToggleButtonTyped.ClickType.FORCED);
            button.SetTrue(button, RUIToggleButtonTyped.ClickType.FORCED);
        }
    }
}