Rankforest tracks and can graph the sales rank of Amazon products over time, with graphs available over all time, 90 days, 30 days, 7 days, or a single day. Many graphs are available for free; some others require a free account, and a final few require paid membership.
There are also hourly and daily RSS feeds for each product.
The site also supports the creation of product collections or baskets, which could be used to track multiple competing or complimentary items.
Finally, there's a simple way to create a "badge" with the current sales rank for any product.