Be happy that your sons have such a wonderful relationship with their dad - it seems like half the posts on here are from mom's whose children don't have a dad involved in their lives, or whose fathers are just horrible people and SHOULDN'T be involved in their lives. Sounds like their dad is a great role model for them.
Do not take it personally. See if there is something that all of you can participate in so you don't feel left out. Maybe talk to your husband about his and how you are feeling. Take them out and do something fun with them just with you (movies, bowling, etc.) Volunteer at their school, or sign them up for Cub Scouts and be a den mother.
Some of what I am reading here is that your entire identity is wrapped up in being a mom to your kids - what did you do or what did you enjoy before you became a mom? Have you thought about what you would like to do once they are older (i.e. middle school/teenagers)?
Do not, for one second, think that they no longer need you or somehow your job is done. They are still really young, just not so dependent right now. They will need you, to some degree, all their lives. They may not remember things as toddlers as well as they will remember things now, but that does not mean it had no impact on them or served no purpose. They are the way they are because of how you have raised them up to this point. My DD is 3 and while she may not remember us taking her to the zoo or the beach or Disney World or reading bedtime stories to her at this age, that doesn't mean we don't do it anyway.
Like some others have said, once they do grow up and are totally on their own, that just means you've done your job.
If you feel like you need to go back to work because you think it might be more fulfilling, discuss that with hubby. But don't go back because you think you are not needed at home. Or think about doing volunteer work that, again, your kids can be a part of with you.
Find a way to enjoy the time that you have to yourself, knowing the kids are happy and having fun with Dad. So many of us don't get enough of that.