Hi everyone ..
Im new here(just wanted to mention this is a great web)
Anyway, big punk fan. Recently started listenning to Sum 41's old stuff
and i wanted you'r recommendation to similar pop punk bands that sound
just like sum 41's old albums (Chuck, Does this look infected?)
If you're looking for good punk check out NOFX, Anti-Flag, Rancid, Teenage Bottlerocket, Guttermouth and The Offspring. The Offspring would probably be closer to the Chuck side of Sum 41, the others are just more common staples in punk.
+1 to what ^ said.

Also Pennywise are a favorite. Some other good stuff... The Deviates, Alkaline Trio's old stuff, Bad Religion, Fugazi, Blink 182 and Green Day's old stuff, Face to Face, Boxcar Racer, Bouncing Souls, New Found Glory, Hot Water Music, Jimmy Eat World, Fenix TX, Kut U Up, Lagwagon, No Use For A Name, The Ataris, Millencolin, Rufio, Rise Against, Yellowcard...

If you want some good old school punk rock definitely check out The Misfits for one and definitely The Ramones. Dead Kennedy's are another good one. Oh and The Descendents, can't forget about them!

That should get you started haha.
the first step to failure is trying.