There are a large number of taxicabs in Porto Alegre. Regular taxicabs can be hailed on the streets, and radio taxis can be ordered by phone. Most radio taxis are air conditioned, somewhat more expensive and experienced at meeting the needs of international travelers. As a general rule, red cabs are the regular less-confortable ones, while white cabs are more confortable (and expensive) drives.
At the airport, the designated airport taxi companies have booths at the arrivals area.
Most taxi drivers don't speak English, so it is advisable to have the address you are going to written on a piece of paper to show the taxi driver.
Tipping is not expected, yet many people choose to round up the fare. Some drivers charge for help with baggage.
You can get anywhere by bus. It's a real complex system of lines and routes and it's advisable to have some local information before trying by yourself.
You pay by bus you take. If you have to take two buses to get somewhere, for instance, you pay it twice. The ticket costs R$ 1,45 (US$ 0.50) and is paid in the bus (no need to buy it in antecipation).
All itineraries and lines can be consulted at the Public Circulation and Transport Service website or by phoning 158. The websytem is quite simple but neither it nor the phone service is internationalized (only portuguese-speakers).
To get to most "touristical" places, you can just ask some english-speaker local which bus to take from where you are. Usually taking the same line back leaves you where you started the trip.
Although a good share of the buses are really confortable, sometimes (mainly during the rush hour) buses get really crowded. Porto Alegre have alternative micro-buses that never gets crowded (it stops picking up passagers if it's full) called "Lotação". Their itineraries are just a little different from that of the buses and many have even the same route names. They can be checked at this website or by 158 phone number. The ticket costs R$ 2,20 (US$ 0.75)
There's no trains or subways inside the city. If you want to travel to other cities in the metropolitan area you can take the Trensurb. The ticket costs R$ 0,75 (US$ 0.25) and is good for one trip (just as buses).