Most computer users who have cable broadband connections lease their cable modems from their service provider. Charter Communications, for example, charges customers $3/month for the entire service period. Sizable savings can obviously be had from buying a modem versus renting.
Amazon.com currently carries Motorola's SB5101 modem for $43.99 (with free shipping and no tax). A little math shows that 15 months of cable service is the break-even point at this price. If you are a new customer, you can often get a large rebate as an incentive for sign-up. Check with your local retailers or ISP for details. If you are an existing customer, setup is easy. Remove the old modem, connect the new, call your provider's technical support and ask to "re-provision" your new cable modem (this does not require a visit from a tech).
While there is certainly a risk to customer-owned equipment, the SurfBoard family from Motorola are quality modems that are likely to save you money over the long-run.
UPDATE (06/19/08): It appears that Frys is hosting a $20 rebate for this modem, making the SB5101 a particularly good deal right now. The rebate form includes Amazon.com as an accepted vendor.
