Garden Planner A non-open source, paid desktop/iOS app for hand-designing a garden and planning your crops. Has a very nice looking calendar.

List of companion plants Wikipedia's list of companion plants. Quite comprehensive, covers some things not in the database yet. Even if you don't want to trust a wikipedia article, it still has links to 50+ useful sources.

Companion Plant Spacing Research done by Kentucky State University on the spacing of intermixed companion plants in a hexagonal formation. The gist is that the spacing is a weighted average of the recommended spacing of the two crops based on the proportions of the crops.

Paper by M.K. Bomford Further development on the KYSU research above. Has a few key conclusions, including:

• Results suggest that mixed planting can increase land-use efficiency of BIA systems under resource-limiting conditions.
• Cool-season crops should not be mixed with warm-season crops for synchronous growth in mixed plantings.

Crop Planning Open-source Java program to plan crops. Not too well-designed or easily usable. Does not appear to have any data on time until harvest, or take conditions into account, or include any companion crop information. However, it's open-source, so it could theoretically be expanded. At first glance it does not seem like a great base upon which to expand.

Another Crop Planner Also open-source, this time a single page application. Angular front, Spring framework Java back end. Claims to offer advice on companion planting, harvesting dates! No longer hosted online, and I haven't yet tried hosting locally, but it is definitely worth checking out if it does indeed do what it claims to. A single page application could be a good format for the companion planner due to its cross-platform compatibility. Also has 970 commits by a single person so a lot of work has gone into it. Also means it's very complex, so more time spent learning someone else's code/system.

Yet Another Crop Planner Open-source planting calendar, this time using Meteor.js. Much more simple, but maybe this means easier to build upon. No companion features but they could be created. After tests, seems to be a very simple interactive calendar (basic legend info on hover) with a search bar and a basic favorites feature who allow you to pin vegetables on the first rows.