There is a posh gym on Terbatas iela, beside the market and the large TV screen / nightclub / casino. Lots of expensive cars in the car park and expensive looking girls not exercising on the machines.
My personal favourite was Atletika, the one just across from the Hari Krishna vegetarian restaurant on Krijāņa Barona iela. Cheap (29LATS per month), with a good selection of cardio,machines and free weights. Various classes too, including kick boxing, spin, and circuits. http://demo.atletika.lv/klubi/centrs/ Website is only in Latvian and Russian. If you want cheaper, you have the community centers that also have gyms.
There is also the gym at the top of Hotel Latvia (now Radisson Blue) where you can run on a treadmill and look over the whole city. I recall you used to get guest membership for 5LATS (7 euros) a session, although its probably different now under Radisson.