There are two sides to this issue, neither of which are unfair. The employer hires people to solve problems in a given space and has a right to make sure they own what they paid for. On the other side the problem space doesn't (shouldn't) include everything. The better defined the problem space the fewer problems you will have. I've been on both sides of the issue and found that people are willing to accept both sides. The last few places I worked management was willing to change the standard contract from the normal "we own all unless you get an exception" to "we own all related directly to this specific project or done on company time/equipment". If your employer is unwilling to make the change and you are unwilling to work somewhere else consider part of the cost of working.