I've just gone through my sites 404 list and redirected all of them to the front page.
There were in general 2 categories of links. The first was relative links of client sites which we may have hosted (we are a web development company), which somehow have ended up relative to our URL. The second were links which were very general, most likely used by spam bots to register on the site (we have blocked all anonymous registration).
My questions is: Should I not have done this? If any of these are spam style links from another site, will redirecting them to my front page actually harm my SEO ranking? How do other users deal with a list of 404 errors?