The best browser is still firefox. There are a number of mac specific browsers you can use, but firefox works for everything.

For putting together split files, I use machacha and think it's fine. There are a number of others, so it's just a matter of preference. Machacha is free, though.

For video, the 400-lb gorilla is VLC. None of the other video players can touch it for breadth of formats, and stability.

All these can be found with a quick google search, and they're all free.