The info below is for version 2.4 and below and thus partly outdated. Updated documentation coming 'soon'.
I hereby present you the PartCatalog.
An elegant tool for a more civilized age.
The PartCatalog allows you categorize your parts into Tags in order to make your building experience less frustrating by reducing the time you're looking for that one part.
Tags are displayed in a toolbar on either of the three screen edges (top one is out thanks to changes back in 0.21).
These tags allows you to filter your parts displayed in the editor part list.
The tags in the toolbar can be further subdivided into subgroups like this.
The shown list is the default automatically created configuration, which is created dynamically based on part types, resources in storage tanks etc.
-Clicking on those tags will filter your part list to all parts included in the tags, those menus are automatically displayed via mouse over, but single top level tags can be enabled via keyboard shortcut aswell.
Use the keys 1-0 for the first 10 tags and shift + 1-0 for the next 10 tags.
-If you have more tags than you can fit on your toolbar you can scroll through it using your mousewheel.
-Holding control while selecting a tag will allow you to select multiple tags at once.
There is a setting whether you want to see all parts included in all selected tags or you want to reduce your filter to only display those parts contained in all categories.
This automatically created tag tree can actually be refined manually, allowing you to reorder your tags, create new ones, merge or delete existing ones.
A sophisticated editor for managing your part tags ingame is included and looks as follows:
It features an ingame context senstive help which can better explain how everything works than I could in a wall of text but an assorted set of features is as follows:
-Sorting of parts in the editor either bei part title, part manufacturer or mod. The latter two will create collapsable categories in the editor.
-Setting custom icons for parts If you want to include a custom icon (templates are included as png and tif files) have the two states one ending in _On.png and one ending in _Off.png, put them to the other icons and they'll be automatically loaded into the icon browser.
-Sorting all top level tags with less than X parts into a seperate small mods top level tag. Useful if you have many small part count mods like MechJeb etc.
-Seperate visbility for tags in SPH and VAB
-Creating custom new pages for certain parts (have all your most used tags in first page, less used parts in the second etc)
Two more usability tweaks are included which can be disabled in the settings if you prefer.
You can scroll through the partlist (as in next page / previous page) and scroll through the categories at the top by holding control.
-Added vertical layout option for small resolutions
-Made vertical layout usable for higher resolutions by control clicking the PartCatalog button.
-Fixed vertical layout persistance
-Automatic Categorization based on part type and subgroups
-Smaller new Features
-Hotfix for 0.22 Compability
-Research compability, tags are filtered out if they only contain unresearched parts (can be toggled in the settings)
-Fixed sandbox troubles
-Fixed not saving layout settings
-Fixed folder structure
-New more random window start id (configurable in cfg aswell incase you get collisions)
-Dynamically assigned categories using lua scripts (easily expandable to cover special mods and the like, I provided a few examples mods to give you an idea)
-New set of category icons provided by toerti
-Automatic creation of abbreviation for mod tags without icon
-Search support (Quick open it using ctrl + space)
-Closing of menus (search, tag editor) using escape
-Properly catching mouseclicks to not trigger changes in your ship when clicking your part menu
-Excluding parts by right clicking tags
-Modifier key support when selecting tags (hold ctrl to multiselect, shift for ranges, alt to toggle all siblings as well)
-Notification that tags are outdated when you install new parts with the option of updating your tags
-Probably a bunch of other new features which I forgot to list
-Fixed GUI transparency for sure
-Part list scrolling works without ctrl key again and for added bonus it locks the camera aswell!
-Fixed tag visibility (SPH/VAB)
-Fixed mouse over delays not persisting (also the mouse over now opens up when you click one of the top level tags)
-Fixed empty tag glitches
-Small mod category is back
-More eye friendly color scheme
-Hopefully fixed file paths for case sensitive file systems
-Fixed shift shortcut for selecting parttags greater than 10 with keyboard.
-Right clicking on the search icon now clears the current search.
-Added a paging option for categories to counter too long lists as a result of many mods, to enable it have a look at the settings page - max subtags per tag slider
-Added configurable startup delay for compability with KSPAPIExtensions.
-Compability patch for 0.24
Download PartCatalog 3.0 RC5 here.
Licensed under CC-SA as included in the download.
This is by no means a competitioner to the great Simple Part Organizer by Mr Shifty, as both synergize perfectly.
I strongly suggest you to have a look at it and consider running both together.
If you have any suggestions or find any quirks please get them posted.
Now go and get tagging!