There are two basic flavors of overdrive commonly used to boost the front of a tube amp: your basic Tubescreamer, and your
SD-1 (asymmetrical variant). The TS tends to smoothen your gain out,
while the SD-1 tends to give your gain more grit. That's the basic gist
of the difference.
On the TS side, you have your Maxon and Ibanez Tubescreamers, which are
the originals. But nowadays there are other derivatives that do the job a lot
MXR GT-OD - most basic; 3 knobs like a regular TS. Digitech Bad Monkey - basic, but has a knob to control bass frequency, which puts it way above a normal TS. Hardwire CM-2 - Basically a higher quality Bad Monkey. Feels better, sturdier, sounds richer/more harmonic. Nice pedal. Way Huge Green Rhino - lots of tone shaping options, this is a very popular pedal, lots of options to dial in exactly what you need.
On the SD-1 side, there's: MXR M77 - Basically just a higher quality SD-1 with more tone
shaping controls. They sound very very similar when used as a boost. I
own both and did A/B comparisons to choose which to use. The MXR won
the spot on my board. Fulltone Fulldrive 2 MOSFET - Very nice pedal with some unusual
tone shaping features, like changing the gain structure a little, etc..
Has a regular OD side, as well as a boost side. AFAIK there aren't any
single pedals with just the OD side, but that side is your SD-1
variant. I haven't tried this one personally yet, so I cant' say much
And that's just some mass produced stuff. When you enter the boutique
range, there are tons and tons of pedals out there. Most of them sound
great, you can wade through them for a while. And then after that,
there's people here who can build you your very own TS to be exactly
what you want. And it could be one of a kind. The TS is one of the
most basic pedals out there, and there really are a lot of alternatives
to Maxon and Ibanez.