For several years now I've been playing with and upgrading a program called Allah Sulu's Massive Tool. (I forget who is to blame for the name.) It has its own community, as_massive_tool, as well as a reference page (now subdivided into categories) showing my vast collection of substitute user and community icons which can be used for customized LJ-type user tags. (This was how the program originally started, and appears to be one of the most popular features.) I welcome suggestions and requests in my community, and have implemented most of those which I have received.
The Massive Tool can be used to post to LJ, from any user account and to any community which the user has set up (and which they also have posting access to, obviously). It will save usernames and passwords; and given those, can also retrieve for a journal the complete list of avatars, tags, and custom friend groups (or just the list of tags, for a community). I'm toying with a rudimentary cross-posting ability (although I'm not sure if I want to include that, or if I should put a limit on it, since it would be ripe for abuse) and I could fairly easily make it compatible with DreamWidth as well if I got enough requests (I know people who use both LJ and DW, but I'm pretty much LJ only). It also have other specific LiveJournal functions, including the ability to create polls; and that is on top of the fact that it's a very powerful and complex text/HTML editor which can create all manner of tables, lists, and other effects. (I use it for editing web pages outside of LJ as well as for writing LJ posts.) It even has a built-in programming language (based on FORTH) which can be used for writing rather complex and powerful scripts.