Opera has had this feature forever, since around 2001. Long enough ago that it shipped with search keys for Altavista and Lycos along with Google. The feature

Opera has had this feature forever, since around 2001. Long enough ago that it shipped with search keys for Altavista and Lycos along with Google. The feature was a primary reason I started using Opera in the first place. Originally you had to dive into a config file to add non-default search providers, but it's now in the UI just like Firefox, right click any text field and select "Create Search".