Backgrounder is actually what allows you to run apps in the background; ProSwitcher is an app switcher that works in conjunction with backgrounded/foregrounded apps.
Kirikae and Circuitous are too other, highly-popular app-switchers. I prefer Kirikae at the moment for its quickness and seemingly small footprint.
For me, Categories/CategoriesSB is the thing to have. You know how Saurik said that changing the background seemed to be the #1 reason to JailBreak? Well, Categories is my #1 thing. That said, it looks like that's one of the things that will be made obsolete by the folders implementation in OS 4.0.
Other must-haves are Infinidock (or, if you really don't foresee needing more than 5 docked apps, FiveIconDock), SBSettings (a status bar swipe that gives you quick access to Settings.app toggles, like BT, WiFi, 3G radio, etc.) and Activator (centralized app invocation settings; let's you pick what motions/actions invoke things like Backgrounder, Kirikae, ProSwitcher, SBSettings, Snappy, etc.).
Oh, another awesome one is Snappy. This is a super-quick-loading Camera app that can be activated by a number of methods via Activator (e.g. press-and-hold on the status bar), even on the lock screen and also allows you to use your physical volume rocker buttons for snapping the pic. Pretty much a must have for the $0.99 or $1.99 asking price.