My must have apps are:
Weather & toggle widget: $1.99 works just like the htc weather widget. the 1.99 is worth it because the developer puts alot of work into his widgets and have many different skins. heres a pic of one of his widgets. $1.99 is worth it for the support you get.
http://s859.photobucket.com/albums/a...0040509112.jpg
pure calendar widget: you will be looking for a calendar widget and this one is the best. There is pure calendar widget and Pure grid calendar, both are $1.99 but pure calendar works best with what we're use to from winmo imo. developer is very supportive, $1.99 is worth it for his work.
gmail unread count: (free app) without this app you will not know instantly how many new gmails you have without clicking the notification bar.
Advanced task killer by Rechild: One of the best task killers and now it includes a widget that will allow you to instantly close apps not included in the ignore list.
Android Mate: A program that give you access to file explore, running programs, and a way to back up apk's incase a certain program is removed from the market or you don't like a certain upgrade that a developer has done to his app, if you have the apk backed up you can install the version you had previously.
and then of course its the regulars like, google sky maps, google goggles, pandora, shazam, speedtest.net app, you know the regulars