Portable freeware works quite nicely with Dropbox I found.
I use several portable applications, which seem an odd bunch if you don't know what I do (*
ahem*) that I have in my Dropbox. I will link them in alphabetical order.
Empty Folder Nuker (I've linked this before, it deletes empty folders, just download and use)
http://simonwai.com/developments/empty_folder_nuker/
FastStone Image Viewer (a very nice image viewer, download the "Portable" version)
http://www.faststone.org/FSViewerDownload.htm
FileZilla Portable (FTP Client, has a little portable installer)
http://portableapps.com/apps/internet/filezilla_portable
FolderSize (tells you which folders are taking up space, unzip and use)
http://www.rotebetasoftware.net/wp/freeware/folder-size/
PDF X-Change Viewer (PDF Viewer. It is not the primary viewer I use but it can export the PDFs as images and other stuff, download the Portable ZIP)
http://www.tracker-software.com/product/downloads
WinDJView Portable (It views DJVU files, which is a bit of a weird format and I havnt seen it very much, but I include in case someone needs this. It has a little portable installer)
http://portableapps.com/apps/office/windjview_portable
X-PDFSam (It extracts pages from PDFs and merges them together and stuff. This one's a bit tricky to install, first you need to copy the English file from the subfolder into the main or you'll get Italian. And then when you launch it it'll install a portable Java so you should be connected)
http://www.winpenpack.com/main/download.php?view.679