Did you upgrade to Lucid and find that some package you like is no longer available? There are a number of binary packages that were removed in Lucid as a cleanup of the archive, though for the most part the source packages are still in there. Most of these were removed because the source package was no longer buildable—meaning patches could not be applied to fix bugs since the patched source would not compile (unless, of course, the patch fixed that build issue). Some were removed because Python 2.5 was removed. To find out what happened to the package you're looking for, try this:
apt-cache showsrc $PACKAGE
This'll tell you the name of the source package (on the "Package: " line). Then go to http://launchpad.net/ubuntu/+source/SRCPKG/+publishinghistory for a full listing of all versions of the package in Ubuntu. If you click the triangle on the left, it'll unfold revealing changelog info for that published version. For example, Sage Math's publishing history shows that it was deleted for the depends-on-Python2.5 reason.
And there's today's lesson on how to find data in Launchpad.