What off the shelf solution/software do you use or recommend for enhancing SharePoint performance?
If you're looking for an off-the-shelf solution, although I haven't used it myself I'm also pretty impressed with what Aptimize does. It performs a series of tweaks such as JS minification, generating image sprites, merging CSS files etc. to reduce the payload of the page.
Microsoft use it on sharepoint.microsoft.com, see How we did it: Speeding up SharePoint.Microsoft.com for a write up.
If you're interesting in doing some of this stuff by hand, I wrote my Checklist for Optimizing SharePoint Sites which discusses some of these issues and other SharePoint-specific steps which can be taken.
There are quite a few ways to increase sharepoint peformance like External binary store and Remote blob store (SharePoint 2010). There are some third party solutions available like NCachePoint, StoragePoint which provide these features.
A good thing abount NCachePoint (http://www.alachisoft.com/ncachepoint/index.html) though is that it has some nice features like js minification, viewstate caching and list caching which really should help improve SharePoint performace.
Avepoint and aptmize are also there but I haven't ever tried. You may find some helpfull features there too.