I shall be visiting and staying with some friends in Dubai, and want to take a day trip out to visit another group of friends in Abu Dhabi. Without hiring/driving a car, what's the easiest way to get from one to the other and back for under 100 USD? According to Wikitravel, a taxi from Dubai seems to only be about 300AED (~80 USD) - are taxis really that cheap in the UAE?
The easiest way is indeed a taxi, and yes, that price sounds about right, although it's a regular metered fare so the exact cost will vary. Not every taxi will be willing to up and off to Abu Dhabi on a lark though, but the airport taxi stand can sort one out if you ask, or you can call and book a cab. As for safe, well... most UAE cabbies drive fairly sanely by the standards of the region, but the Dubai-Abu Dhabi highway always has the occasional blinged-up rich kid doing 300km/h in the fast lane in Daddy's Lamborghini.
The cheaper and safer, but slower and less convenient, option is the Emirates Express bus, Dhs 20 one-way. Buses leave from Ghubaiba station, on the Dubai Metro Green Line, every 30 min or so.