Last night I made a simple pesto with the leftover opal basil (i.e. the purple kind) from the farmers' market. The only problem I had was that I couldn't find any pine nuts at the local market... not to mention that the last time we made a dish with pine nuts Anna got pine mouth (nuts were from Trader Joes and imported from various countries in Asia)... so instead of running all over town and getting more pine nuts of dubious provenance, I decided to try a substitute. I could have just made pistou, which is essentially pesto without nuts, but I decided I wanted to see what the alternatives are like. A perusal of the internet suggests (like this Serious Eats thread) that basically any nut can be substituted, but walnuts, almonds, and pistachios seem the most favored. According to Saveur's latest issue walnuts are the most traditional choice (since there is a Northern Italian variation using them), but that you should either lightly toast or soak them in water (for at least an hour)... to remove the bitter qualities of the raw nuts. Unfortunately I did not see that issue of Saveur on my coffee table (with a big "Why we love pesto!" headline) until after I made mine sans toasting or soaking... whoops!
Well I definitely recommend doing one or the other if you are substituting for any reason... the bitterness was noticeable, if not overwhelming... so I'd definitely do it in the future. Oh, and the recipe I used was Simply Recipes... not that I'm sure you really need a recipe for pesto, but I like the security blanket.