Alone the Doc Johnson Spectra Gel is my favourite, so I'd like something similar, maybe with the different bubbles/ridges. But also something a bit bigger, but probably not longer.
It seems my request is quite specific but having read the forum for a few months now I figured you bunch would be able to point me in the right direction!
...you're obviously a butt-plug fan - if you're the type that likes, or would like, to wear one outside the bedroom I really can't recommend http://www.lovehoney.co.uk/product.cfm?p=9042 highly enough - I know it's pricey but it'll last you for life (I...um... have mine in right now )
I concur with bi-cyclist on inflateables - Used to have one &, while it provided a good filling sensation, the "stretching" wasn't really there due to the softness when inflated.
And finally if you want something cheaper (yes, I realize all my recommendations are a bit expensive - what can I say? I have expensive tastes!) - perhaps just go for a good sized vibrator. These days me & Ms SWW have an extensive toy collection, but for a long time when I was playing alone all I had was a couple of plugs and a good 6" vibe and I was happy as Larry.
Don't forget to let us know what you go for! Happy Shopping!