The easiest way to get hotmail onto an iphone is to pay Micromong 15 quid and register your hotmail account as paid. Then it's easy to put it onto the iphone.
As for MSN, the best app is (through trial and error) Beejive although it is expensive. The others (palringo, fring, etc - are toss).
Weatherpro - includes a rainfall radar (aviators can get great free TAF and METAR etc on AeroWeather).
Myrail Lite - see your train - and whether it is on time or not. Stay in bed if it's late.
Ebay - free app; great functionality.
Shazam - music recognition - amazing and uncannily accurate.
Seadragon - get this app whilst it is free because the technology will be at the heart of a lot of stuff soon....
iTalk - record long meetings.
RK_Free - running app. Great but you have to tune the settings.
Grid - gives you OS Grid references instead of Lat/Long
Clinometer - a very professional app - a good benchmark of what ipod apps should look like.
Panolab pro - great photostiching app. Has a free version too.
Boomberg - watch the country go down the swannee in greater detail than the native stocks app...