Framework 2.0 in poi:

codice:
<asp:Button ID="btn" runat="server" Text="Esegui Javascript" OnClientClick="alert('ciao, mondo'); return false;" />


oppure:

codice:
ASPX:
<head runat="server">
    <script type="text/javascript">
        function ciaomondo() {
             alert('ciao, mondo');
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Button ID="btn" runat="server" Text="Esegui Javascript"  />
    </div>
    </form>
</body>
</html>



VB.Net
    Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Init
        btn.Attributes.Add("onclick", "ciaomondo();return false;")
    End Sub