Navionics has a nice interactive web mapping service that's been online for a few years, you can pull up contour maps for your favorite lakes and rivers on your computer. I've found it very valuable for pre-fishing lakes: https://webapp.navionics.com/?lang=en#@11&key=%7Bgk%7CGp%7DixP
There's also an iphone/android app that puts these Navionics contour maps on your phone or tablet, about $10 per year for SD and $50 per year for HD.
What I've found very valuable on Navionics is their SonarChart app that takes depth+GPS measurements from fishermen and updates the contour maps, this is very valuable if you have a lake / river that's not currently plotted or you find structure in a lake that's not on the current maps.
I've used SonarChart to chartplot the small "uncharted" lake behind my house, there's instructions on their website how to setup your GPS-enabled depth finder to record data onto a blank SD card and then upload the data to Navonics. I did this with my Humminbird depth finder and the "uncharted" lake showed up on the Navionics web app about a week later.
I've also used the Navionics SonarChart Live real-time data colleciton and uploading, in my case I have a Vexilar TBOX (wifi depth finder) in my boat and an android tablet. The Navionics Boating app auto connects to the vexilar depth finder and records depth from Vexilar+GPS data from the tablet real time. This past weekend I ran up the prairie river a few miles, this was uncharted territory last week but now I notice on the webapp that the river has been charted.
I don't work for Navionics, I've just found these tools to be very valuable so I wanted to share them in case you haven't heard of them yet. My lake chip is a LakeMaster but I must say that this new stuff from Navionics is pretty innovative and valuable. You no longer have to spend a $100 on a chip every few years, instead you can pay $10 per year and get the Navionics maps on multiple devices and the maps are constantly being updated.