You don't mention exactly what work you do, but a good DSO will last you a while (and I guarantee it won't go out of date for a while). Sure, you can grab a Tek analog scope 2nd hand but my storage scope has saved my frustration bacon more than once. I have a Rigol DS1052E (a good review from a proper EE here) which is a 50mhz 1GSa/s scope direct from the factory; they are about 675$AU on ebay. In my opinion they are the best high end hobbyist scopes on the market; considering what you get for the price I wouldn't trade mine for anything.

I never had any luck finding a good used 'scope on eBay or Craigslist; there are tons out there, but they're typically missing probes or documentation. I ended up buying an MSO-19 from Link Instruments for about $250. It's a computer-based USB single-channel digital storage oscilloscope and 7-channel logic analyzer. It works well on Windows XP running in VMWare Fusion on my MacBook Pro running Snow Leopard. I have not found any USB-based 'scope with OS X-native software. I find the fact that it's a single-channel scope to be a limitation: it'd be really nice to compare a raw signal to one that's been processed in some way (through an RC filter or op-amp), but it's proven to be extremely useful.

Parallax has two oscilloscopes, the $250 PropScope, and $140 USB Oscilloscope. I don't have any experience with either of these. They do appear to both be 2-channel scopes, but neither seem to have any kind of logic analyzer.

Do you need the analog oscilloscope features, or are you planning to use it more for digital logic analysis? There are some decent PC-based logic analyzers that you can get for very cheap considering their capabilities, if you don't mind forgoing the ability to do analog signal analysis.

As far as actual analog-capable PC-based oscilloscopes, this is the only one I know of that has decent capabilities at a reasonable price: Link Instruments MSO-19. It actually combines 1 channel oscilloscope with an 8-bit digital logic analyzer, so you're getting a good set of features. Supposedly it can sample the oscilloscope at 2GHz, and the digital logic analyzer channels at 200MHz. It's mentioned by another poster above, for $250.

If you just want a digital logic analyzer, the best low priced one I've found is the Zeroplus Logic Cube LAP-C 16032 with a 32kbit/channel buffer, 16 digital channels, and can sample up to 100MHz, it's only $120.

Since no-one has mentioned it yet, I'll post the link to the afrotechmods guide on purchasing an oscilloscope thinga-ma-jiggy.

http://www.afrotechmods.com/groovy/oscilloscope_tutorial/oscilloscope_tutorial.htm

first video on that page is the discussion about what kind you should buy etc.

Just get a used one on eBay. It's much cheaper than a PC scope. My family got me an old "Lizen" 20 MHz scope, which works well enough for what I do. I suspect it was under $50.

There's also this thing for $89, if it meets your requirements: http://www.seeedstudio.com/depot/micro-digital-storage-oscilloscopedso-nano-p-512.html

I have a BitScope, because I saw one for cheap on eBay, but I rarely use it.

There's an article in the January edition of Elektor Magazine about building your own multi channel Logic Analyzer based on ATM18 board that they sell(?). This in turn is based on an earlier article about building your own Storage Oscilloscope using the same board. Depending on your requirements, either of these might be suitable as a stop gap until you settle on a final one.

You appear to have about a week to get to WHSmith's before the next issue comes out, alternatively you can pay about £1.10 to download the article about the Logic Analyser here or the Storage Oscilloscope here (you need to buy Elektor credits from the left hand menu). The board it's based on appears to be here.

I've got a dual trace PC based one made by Parallax. It has very limited capabilities because it's limited to the speed of the USB bus while taking samples. This makes it easy to use when you have a laptop or desktop PC handy, but also makes it useless when working on any higher speed signals. In all I only spent about a hundred $ on mine and it has served my prototype and hobby needs quite well. It just isn't any good for radio or microwave work.

Owon standalone oscilloscopes are pretty cheap (25MHz less than $200 if ordered from the manufacturer) and quite good for their price. http://www.owon.com.cn/eng/pdsSeries.asp