Citeseer contains a list with the estimated impact of publication venues in Computer Science up to 2003. The parameter that determines impact is the average citation rate over all articles in a given year. Self-citations do not count.
Microsoft Libra seems to be more up to date (at this moment until 2006) and with many more interactive features. It supports ranking papers/authors/conferences/journals. The fact that it does not normalize citations over all papers seems to give better rankings.