A couple of old Archos devices which are now often found very cheap on ebay, even new sometimes:
Archos A43IT gen8 16GB (4.3 inch screen), as already mentioned on the first page. Mine is rooted - see http://dev.openaos.org/wiki/SettingUpMultiRootGen8
- and running an otherwise regular Archos 2.4.83 firmware (Android 2.2.1 Froyo). It runs Rasher's 480x800 build of March 1st 2014 and works without problems of any kind. The theme I use is a modified version of DFKT's Minimum 480x800 (see pg. 1 of this thread). It is really excellent for touchscreen use and easily modified for tweaking metadata display and similar. This is a really good device for Rockbox as the screen is big enough to be comfortable, sound quality is great so long as you don't use multi-driver IEMs. If you use headphones with a microphone and a four ring jackplug you may need an adapter (a plain extension cord can do the trick if you don't mind losing the mic). Battery life on audio playback is very long indeed (think days not hours) even with a microSD card inserted.
Archos A28IT gen8 4GB: (2.8 inch screen) Also running Archos 2.4.83 firmware (Android 2.2.1 Froyo) but not rooted. Rockbox version is Rasher's 240x320 build of 20th March 2014. Everything works fine. For a theme I use grayfog from http://themes.rockbox.org/index.php?allthemes
(I use the same theme on my Fuze+), again modified for metadata display of comment and composer fields. The controls are very good, with volume up and down being mapped to top and bottom of screen and pause/stop mapped to the centre so you don't need dainty fingers despite the small size. It's a resistive touchscreen so you could probably even use it quite well with gloves. Like the Fuze+ The screen is big enough to make a good job of displaying cover art and info. Sound quality seems fine with the same provisos as made about the A43. The output level seems a bit lower than the A43, though I didn't do a level match or test yet. Battery life is nowhere near as good as the A43. With a small disk capacity and no microSD slot I am much more likely to be playing back files from local shares but unfortunately the battery is tiny (680 mAh) and wifi use drains the power quite a bit. Power saving is more aggressive on the A28 than the A43 and not configurable so you might even experience interruptions if playing back files on network shares. I didn't do a proper measurement yet but I would be surprised if it achieved more than 10 hours unless wifi is off. One positive point is that the device is tiny, only about 15mm longer than a credit card and the same width. It's possible to attach a very slim credit card size power pack to it and still have the benefit of a tiny device.
Both devices support pause/resume on headphone being unplugged/attached.
Both devices have excellent codec support even without Rockbox, so if you want to run MPDroid and playback streaming wav or flac from your mpd server instead of the usual ogg or mp3 then these are great, especially the A43 with its big battery. You can also do this over ssh tunnels so when away from home you can enjoy secure passwordless remote access to your home media storage (VX ConnectBot is great for this).
If you look at these as extremely versatile networking multimedia players and not full tablets then they are actually excellent. If you expect an iPad or Galaxy Tab or any kind of modern tablet you will be horribly disappointed. If you want Rockbox + benefits these are worth a look. They are old and cheap now. I bought an A43IT for quite a lot of money in 2011 when it was a new product line but clumsily broke it beyond repair. Recently I bought my current A43 boxed and as good as new complete with the AC3 plug-in and leather case on ebay for £45 and my A28 boxed and new for £25 on ebay.