Will any of RIM's software work without a blackberry data plan?(but with a carrier data plan)
I know that push email or bb browser will not work, but what about Yahoo Messenger or other great apps?
Also I want to know if I use a third party application like Opera Mini, will I be abble to view atachaments from gmail?

You need BIS or BES to run any RIM apps.

If you have generic data plan i recommend jive talk for instant messaging, opera mini for web browsing (it does use lots of data, so be careful) and the gmail mobile application. With the gmail mobile, you can view attachments, but cant save them.

Never understand that question or why have one without the plan.

yes you can
i have just installed oprea mini and as i had a internet allowance in my monthly plan, i can browse the web. although you will need a data plan for things like BB messenger and push email. but if you have a web based email then opera is gr8.

cheerio

@NCBerry
Because I bought a cheap one from ebay and a bb data plan is more expensive than the phone )
@technovice
Thanks for your answer. Is any free IM application that works without data plan?

Yahoo! Messenger on your mobile. Message instantly—anywhere.

Try the above. I think you can set it to use text messaging.

You could also maybe use Google Talk on the web.