Limp aims to be a fully featured Common Lisp IDE for Vim. It defaults to SBCL, but can be changed to support most other implementations by replacing "sbcl" for your favourite lisp, in the file /usr/local/limp/latest/bin/lisp.sh
When discussing Lisp these days, it is commonly assumed to be Common Lisp, the language standardized by ANSI X3J13 (see the HyperSpec, and Practical Common Lisp for a good textbook) with implementations such as GNU Clisp, SBCL, CMUCL, AllegroCL, and many others.
Back to Limp. There are other solutions that are more light-weight, or try to do other things, but I believe in providing an environment that gives you things like bracket matching, highlighting, documentation lookup, i.e. making it a turn-key solution as much as possible.
In the Limp repository you'll find some of the previous work of the SlimVim project, namely the ECL (Embeddable Common Lisp) interface, merged with later releases (7.1); Simon has also made patches to 7.2 available yet to be merged. The ECL interface is documented in if_ecl.txt.
Short-term work is to do said merging with 7.2 and submit a patch to vim_dev to get it merged into the official Vim tree.
Which leads us to the long-term plans: having Lisp directly in Vim will make it convenient to start working on a SWANK front-end (the part of SLIME that runs in your Lisp, with slime.el being the part that runs in the editor - the frontend).
And somewhere in between, it is likely that all of Limp will be rewritten in Common Lisp using the ECL interface, making Limp easier to maintain (VimScript isn't my favourite) and being easier for users to customize.
The official Limp site goes down from time to time, but as pointed out, the download at Vim.org should always work, and the support groups limp-devel and limp-user are hosted with Google Groups. Don't hesitate to join if you feel you need a question answered, or perhaps even want to join in on development. Most of the discussion takes place on the limp-devel list. If you're into IRC, I'm in #limp on irc.freenode.net as 'tic'.
Good luck!