I'm in Buenos Aires for a couple of days and I'd like to go to Montevideo for a day trip. I've seen that there are ferries and planes between the two cities, but are there also other options? And which of these options is normally the cheapest and which the fastest?
There are regular and express ferries from Buenos Aires to Montevideo.
One option goes directly to the Ciudad Vieja district of Montevideo, situated very close to downtown, with the Buquebus ferry company, and takes about three hours.
Another option is the ferry+bus combination (which I've personally done in the reverse direction), which has two companies - Buquebus and ColoniaExpress, which takes you to Colonia, and then down by bus. It's cheaper, but takes an extra 1-2 hours depending on crossing times and connections.
There's actually really good information and links on Wikitravel's Montevideo article.
By ferry, there is also a third company (SeaCat) which make the Buenos Aires - Colonia crossing and their rates are generally the cheapest of all three companies. I agree Mark, however, that you'll leave yourself quite short on time if you go via Colonia.
If you fly, since the former Uruguayan airline (Pluna) closed in 2012, your only options are LAN/TAM or Aerolineas Argentinas. Prices aren't too bad but you will struggle to do this as a day-trip, I feel - once you've allowed for international check-ins, etc. you'll only end up with a couple of hours in Montevideo. For example, for a random date in October, the earliest you could arrive by air in MVD is 10am, and you'd have to be back at the airport for check in at 4pm.