Just about every guitarist has heard of the green monster the Ibanez Tube Screamer. The most famous model for it's awesome creamy tone is the TS808. It was used and made famous by blues guitar legend (SRV) Stevie Ray Vaughn and others.
This brief history shows all the TS series Ibanez pedals to date including reissues and the different components that where used.
The first Tube Screamer is the venerable TS-808 Overdrive Pro, introduced somewhere around 1979-1981 There were predecessors (OD-850 and OD-855), but the 808 was the first one to be named tube screamer, so let's start from there. All of the TS-808s used either the JRC 4558D or the TI RC4558P chip. These pedals are very expensive these days, unless you come across one at a garage sale ebay etc..
Around 1982-83 Ibanez introduced the 9 series, with a completely redesigned exterior, and the TS-9 Tube Screamer was born. The main differences from the 808 were a few resistors in the output section, different pots (design, not value) and op-amp chip. A few TS-9s had the JRC 4558 chip, but most were shipped with either the JRC 2043 or the TA 75558. Original TS-9s are starting to get real expensive as well.
Note: The 9 series also contained the TS-9s bigger brother: the ST-9 Super Tube Screamer. Essentially a TS-9 with a mid boost control, painted dark metallic green. Not to be confused with the later TS-9 DX. The TS-9 circuit was also used in the UE-300 mains-powered floor multi-effects.
Just a few years later in 1985 the 9 series gave way for the Master Series - which looked just like the more well known 10 series, but with mostly black casings. Interestingly enough, Ibanez didn't carry the Tube Screamer over to the Master Series - they kept the oh-so-rare Super Tube Screamer instead, and called it Super Tube (STL). In 1986 Ibanez replaced the Master series with the Power Series, which is the one also known as 10, and the TS-10 Tube Screamer Classic was born. The TS-10 can be had for a lot less than a 9, and will sound close enough, if not just as good.
They are harder to mod to 808 specs, though, as the TS-10 not only had component value changes (like the 9) but also several new components that weren't there before. The input/output jacks are mounted directly on the pcb, with no support from the box, and will eventually fail. It's still a great pedal, and not too expensive. You might even get it cheaper than that, if you find one that's broken. Nine times out of ten, it's the jacks that have failed - if you remove them carefully, you'll be able to glue them on to the pcb before you solder them back in. Fixed! BTW, this construction weakness (or bargain opportunity, depending on how you see it) goes for the whole 10 series.
The later Sound tank series (the one that was supposed to look like a tank, but mostly reminds me of a bug) also had its version of the TS: the TS-5 Tube screamer. Apparently (I haven't heard one myself) it sounds darker than the others, and therefore not as appealing. They are cheap enough, though, so buying one just to have it won't clean you out. It has the same weak spots as the 10 - pcb-mounted jacks that just poke out of holes in the enclosure, without support.
This one also has its pots mounted the same way, plus the enclosure itself is plastic... Apparently, in its first incarnation the Sound tank series had metal enclosures. The series then consisted of six different od/dist/fuzz boxes, and nothing else! The second time around, when the TS-5 first appeared, they switched to plastic. I guess this is why you don't see many Sound tanks around - they're all busted up.
Even later (and still in production, I believe) is the TS-7 Tubescreamer. This is the one with the "gain" switch and the funky pop-up pots. From what I hear, this one is pretty good, and easy to mod as well. It belongs to the Tone-Lok series (no doubt referring to the pots being locked down when not in use). The TS-7 has a JRD4558 op-amp (instead of the JRC version) which apparently doesn't help the pedal very much. Other than that, it's a good one.
Reissue TS-9 Tube Screamer, which is a more or less exact copy of the early 80's pedal and in 1998 the TS-9 DX Turbo Tube Screamer appeared - a TS with three added modes. Both of them use the TA 75558 chip exclusively. In 2002, Ibanez took over the production of these pedals from Maxon - possibly because Maxon decided to reissue the 9 series on their own.
The latest model is the reissue TS-808, which basically is a RI TS-9 with an 808 style case. It uses the same circuit board, pots and components as the TS-9, and apart from the cosmetics, the only differences are the two output resistors (which are changed to 808 values) and the op-amp chip (current production JRC4558D).