The notion of a priori changed a lot since Kant, see Did Kant consider Newtonian mechanics a priori? Today they are seen as potentially fallible even if not empirical.
The Austrian school, including Brentano’s pupils Stumpf, Husserl and Reinach, and more recently "Manchester three" Mulligan, Simons, and Barry Smith, focused on more immediate and elementary a priori. The idea is that they are a priori because when we try to conceive of a counterexample not only can we not, but we "see" that it is impossible. Barry Smith wrote an interesting essay In Defense of Extreme (Fallibilistic) Apriorism arguing that attempts to do without such a priori invariably end up relying on them in a guise. Polish philosopher Wojciech Zelaniec catalogued a list of prototypical examples of Austrian a priori (see below).
As for more "serious" a priori, the conception was developed by some neo-Kantians (Cassirer, recently Friedman) and logical positivists (Reichenbach, Carnap). These are also fallible and revisable, but they are by no means obvious, and their discovery may require a lot of work and thought. Although they may be established in empirical sciences it does not make sense to call them empirical, because they need to be assumed to enable empirical measurements and their theoretical interpretation to begin with. Friedman classifies them into "coordination principles" connecting theoretical parts of scientific theories to observations (e.g. the law of inertia in classical mechanics, or the equivalence principle in general relativity), and "philosophical meta-principles", that act as extra-empirical selection rules (like locality, causality, gauge invariance, general covariance, etc., in physics). See his Einstein, Kant, and the Relativized A Priori and Dynamics of Reason.
Stjernfelt's Diagrammatology (2007) has a nicely written review chapter on synthetic a priori. Zelaniec's list is quoted from there, names in parantheses are philosophers that discussed the example.
Examples of Austrian a priori
- every color is extended (Kant, Berkeley, Hume, Husserl, Stumpf)
- for every two events, if one of them is later than the other, the other is not later
than the first one (Pap)
- if something is beautiful and real, then it is good (Roth)
- everything red is colored (Chisholm)
- every three tones are ordered linearly with regard to their pitch (Roth, Husserl,
- the pleasant is preferable to the unpleasant (Scheler)
- no surface, if it is red all over, is at the same time green all over (Schlick,
Wittgenstein, Russell, Ayer, Pap, (Aristotle))
- everything that is square has a shape (Chisholm)
- only good actions can be the object of a duty (Scheler)
- man acts (Hoppe, von Mises, (Aristotle))
- if any tone-quality is eliminated, a tone-intensity will also be eliminated
- every promise gives rise to – mutually correlated – claim and obligation
- pink is more like red than black (Austin, similar examples in Locke, Hume,
- every judgment comprises a presentation within itself (Stegmüller, (Brentano))