SLIME is available from the CVS repository on
common-lisp.net. You have the option to use either the very
latest code or the tagged FAIRLY-STABLE snapshot.
The latest version tends to have more features and fewer bugs than the
FAIRLY-STABLE version, but it can be unstable during times of
major surgery. As a rule-of-thumb recommendation we suggest that if
you follow the slime-devel mailing list then you're better off
with the latest version (we'll send a note when it's undergoing major
hacking). If you don't follow the mailing list you won't know the
status of the latest code, so tracking FAIRLY-STABLE or using a
released version is the safe option.
If you checkout from CVS then remember to cvs update
occasionally. Improvements are continually being committed, and the
FAIRLY-STABLE tag is moved forward from time to time.