Auction monitoring is often included in other applications where the emphasis is often on the primary function which might be searching or sniping. Monitoring ends up as a separate screen in a larger application and in my own (limited) experience, is not particularly flexible. With eBay Item Watcher we have a small application that monitors auctions extremely well. It's flexible in that you have a fair amount of control over how EIW keeps you informed of auctions that interest you. The interface is a clean and uncomplicated list. To add an item you paste the URL of said item into the Add dialog. The unregistered version can display 4 items only. There's the usual array of columns that provide all the info needed for monitoring. They can ALL be sorted or hidden as you wish. Hovering your mouse over an item brings up a balloon with all the info. Double clicking on the item brings the listing up in your browser. The refresh interval can be set to suit your requirements. The maximum is 2 minutes which is a bit too frequent for my personal tastes. I usually monitor at 5-10 minutes and switch to 1 min for items about to end. Watched items that are about to end can be highlighted. The time period can be set between 1 and 30 minutes before end. Alerts take the form of a message box and/or a balloon (new bid, auction end, etc). Hopefully a definable sound will be added as an alternative too. There is also an option to export information in csv format which will allow you to keep records (if you wish) in a spreadsheet.
Finally the price. Currently the registered version is 1GBP! (~$2) payable through paypal. This is amazing value for money and I doubt it will last. At that price, I recommend you buy a license just in case you might need it sometime. If you can't tell.. overall, I'm very impressed. Please support this developer.