Con-artist taxis do a lot to hurt the image of Bucharest, and Romania, and word-of-mouth of travellers crossing Eastern Europe often echoes the rip-offs that start at the Bucharest train station. Many new arrivals walk out of the Gara de Nord (Bucharest's main train station) and find goons plugging yellow taxis charging exortionate (though slightly legal) rates to get you to your hotel or hostel. I've heard of some visitors paying $150 for a $5 or $10 ride!
Try to avoid these guys at all costs. They're pretty much all jerks.
If you have access to a phone, you can call in a taxi and wait for it. A couple reliable companies include Cobalcescu (tel 021-945), Cris Taxi (tel 021-9461), Taxi Sprint (tel 021-9495). If you don't have a phone, Wasteels (021) 317-0370 is a travel agency inside the train station's arrivals hall; they speak English and often will help you call a reliable taxi.
It's relatively easy to get around by metro too. The Gara de Nord metro station is connected. Tickets are available for a couple Romanian lei, though transfers are not always the easiest to figure out. Many young people in Bucharest speak English, and are quite happy to help you find your way.
Buses outside are another option, though you will have to track down a (usually gray) ticket booth to buy the 1.10 lei ticket. Booth attendants are less likely to speak English, and you'd need to know what bus to take. Ask your hotel in advance.