Boss TR-2

There are a lot of more expensive tremolo pedals out there, but this is in my opinion best bang for the buck, especially if you just want to try out a tremolo pedal and don't really know what to buy.

Tremolo pedals on general have a bad reputation of being noisy and/or eating up your signal. This one doesn't really have any volume drop, but it is noisy a bit, especially if you use a big valve guitar head (you can hear how your signal is faintly pulsating even if the pedal is not ON). Make sure you use a dedicated PSU for this pedal, or a good quality power supply with isolated outputs, and you will minimize the pulsating of your signal. That was my experience with the pedal, at least.
If this pedal had tap tempo, I would have given it more stars, but then it wouldn't be relatively cheap as it is.
Anyway, because it's not any kind of boutique pedal, you can probably try it out in your local guitar shop, so if you're interested in getting into tremolo pedals, I would definitely recommend trying out this one.

I've had this pedal for a great while now and I really enjoy it. As always, Boss' build quality is amazing and I would think that this pedal will outlast all the other pedals on my board since it's built like a tank. However the LED isn't very bright and that might be a problem for some people. Lucky enough it is very easy to replace.
The TR-2 is an underestimated pedal. It can give you all kinds of cool tremolo sounds without having any fancy features like tap-tempo and hundreds of waveforms. However I think it can be pretty hard to dial in the exact speed you want. With the speed knob at 12 o'clock it is really fast and beyond that it is almost unusable to me. Also the speed knob works in conjunction with the wave knob, so if you think you have found a good speed but you want a bigger wave you will probably have to adjust the speed again afterwords.
If you have never used tremolo before this is a great pedal for you. It comes with suggested settings if you do not know how to set it up, so you don't feel like you are fumbling in the dark with the settings. Overall this is a great, vintage sounding and cheap little tremolo pedal that I highly recommend if you are looking for a simple set-and-forget type of tremolo sound.
Enjoy!